Due to the scarcity of energy such as gas, oil, and electricity many heating and cooling systems have been developed which make better use of our resources. Some of them employ a heat exchanger to generate hot water. Other systems have been developed which make better use of our resources to heat hot water. However all of these systems require complicated and costly interconnections and relatively large space. Furthermore the existing systems do not make special provisions for intermediate periods when no space heating or cooling is needed, only hot water. Heat pump water heaters are only available as independent systems with a small compressor.